Recent evidence suggest the existence of two major subpopulations of murine thymus derived (T) lymphocytes. These populations can be distinguished not only by differences in the relative density of the T differentiation antigen theta present on their surfaces, but also by differences in their relative responsiveness to the T dependent mitogens phytohemagglutinin (PHA) and concanavalin A (Con A), differential homing patterns, differential radiation sensitivities, and differential functions. The aims of the proposed research are to further investigate the ontogeny and function of these T cell populations. Each population will be isolated in vivo utilizing their differential homing patterns. The subsequential differentiation and maturation of the populations will then be followed. Moreover, the isolation of each T cell group allows studies of their function as regards mixed lymphocyte reactivity, delayed hypersensitivity, and in vitro killer cell activity. These studies will not only be useful in advancing our knowledge of T cell maturation and differentiation, but also in elucidating the functional roles and interactions of T lymphocytes.